1. Field of the Invention
This invention is directed to microwave antennas, in general, and, more specifically, to antennas which are mounted on a movable platform (vehicle) the motion of which tends to cause the polarization and beam pointing to become disoriented with respect to a required transmission path. This invention achieves dynamic polarization and beam stabilization so that path alignment requirements can be met.
2. Prior Art
There are many antenna systems known in the art. These antenna systems can be used in radar systems or the like and can be used for tracking and/or signalling. Most of the known antenna systems operate on a rotating basis to provide both the azimuth and elevation variable. This two-axis antenna system is usually arranged to be supported on bearings and driven by a motor-gear-train apparatus. Thus, two degrees of rotation are achieved.
In the past, in order to compensate for variations in the operation of the antennas, circular polarization of the signal has been required. However, this signal configuration tends to cause significant problems in the generation of the signal, as in the well as interpretation of any response signals. However, in the past utilization of linearly polarized signals has been precluded inasmuch as an operational third degree of rotational freedom has been impractical. Typically, the circularly polarized signals which are currently used tend to be relatively easy to separate from noise and other background signals while, also, producing little or no problem in terms of reflection signals. These are advantages of using circular polarization, over linearly polarized signals. That is, with linear polarization, the signal produced by existing equipment produces variable response signals and, as well, produces stray signals and the like.
It has been recognized that in order to utilize linear polarization, it is necessary to provide a stabilized antenna. This requirement is usually dependent upon a stabilized antenna platform. With this apparatus, it is possible to keep the signals which are generated by the antenna at right angles (i.e., linear polarization) without having the problems noted above. Also, this would avoid the necessity for utilization of circular polarization signals.
On the other hand, stable platforms have been determined to be extremely complex and cumbersome in terms of being relatively large and heavy. This is a distinct drawback in many applications. As a consequence, use of linear polarization for dual channel isolation has been largely ignored in airborne systems, in general, and airborne antenna systems, in particular.
Many antenna systems utilize two-axis rotational motion. Typically, there are independent elevation and azimuth axes to permit complete spatial coverage from the horizon to near zenith at all azimuth angles--with allowance for pitch and roll movements of the vehicle. These two-axis antenna systems are usually arranged to be supported on bearings and driven by motor-gear-train apparatus. A servo system is included to realize positive control based on the required beam pointing information. Usually, there is no provision for a third axis of motion. Even if provision for a third axis is implemented, it is not dynamically controlled and is not referenced to a fixed spatial coordinate.
In the past, problems associated with polarization misalignment were reduced by the use of circular polarization. With circular polarization, there is no signal loss due to a relative tilt between two antennas as there is when linear polarization is used. Circular polarization also provides some relief from multipath effects at extremely small elevation angles. However, circular polarization is more difficult to implement than linear polarization.
In the present invention, the objective is to increase the use of a communication channel by using two orthogonal polarizations, simultaneously. Orthogonal signals do not couple together whereupon two independent signals can be transmitted on a single channel. The orthogonality can be realized by using either right- or left-hand circular polarization or dual linear polarization with each signal oriented in spatial quadrature (90 dgrees relative spatial position). Linear polarization is readily implemented without low tolerance requirements. However, for a variable attitude platform, linear polarization must be stabilized spatially. Circular polarization is very difficult to implement and requires very low tolerances.